METHOD FOR MANUFACTURING FIBRE COMPOSITES

Abstract: A method of manufacturing bodies of polymeric fibre composites, wherein a preformed fibre reinforcement (11, 21, 31, 41, 51) is placed in a cavity whereupon a liquid resin of a low-viscous polymer material is supplied to the cavity through one or more injection channels (13, 23, 33, 43, 53) provided in the fibre reinforcement, whereby the resin supply through the injection channel is controlled such that the flow front (19, 29, 39, 49, 59) of the resin during the mould filling propagates parallel to the axis of said injection channel through the cavity such that the fibre reinforcement is impregnated by the polymer material, the viscosity of the polymer material then being increased and the body being stripped.

Description: Method for manufacturing fibre composites TECHNICAL FIELD The invention relates to a method of manufacturing polymeric fibre composites by injecting a polymer material in a forming tool, a mould, comprising a prelocated fibre reinforcement which is usually called Resin Transfer Moulding (RTM) , Injection, Resinject or Liquid Composite Moulding. Herein- after, the designation RTM will be used in this application. More specifically, the invention relates to a method by means of which a good and ensured mould filling, without air inclu¬ sions or other unwanted inhomogeneities, is obtained while at the same time the fill time is considerably reduced. BACKGROUND ART RTM is used for manufacturing a large number of polymeric composite products. The process can be used for both large and small objects, for products with high performance requirements and regardless of geometrical complexity. Further, the flexibility of the process is great with respect to the material choice for both fibre reinforcement and polymer matrix. Common matrix materials are thermosetting resins such as unsaturated polyester and epoxy. All common fibre types which are used for the purpose of reinforcement, such as glass fibre, carbon fibre, thermoplastic polymeric fibre materials, ceramic whiskers materials and metallic fibres can be used during the process. In simplified terms, the RTM process can be described to comprise the following steps: 1) placing the fibre material in a forming tool, a mould, 2) locking all mould parts, 3) filling the mould by injecting a liquid polymer into the tool through one or more gates, whereupon the tool is dea- erated through one or more deaeration points, 4) curing the polymer material in the mould, and 5) opening the mould and stripping the formed part. The process places demands on the physical properties of the polymeric matrix material and on a good control of its change during the curing stage. The mould filling requirement means that the liquid polymeric matrix material must exhibit a sufficiently low viscosity and maintain this until the whole forming tool (mould) is filled in spite of the fact that the curing sometimes is started as early as during the mould filling stage. At the same time, the formed part should rapidly reach a stiffness after completed mould filling, in that the curing reaches such a level that the part can be removed from the tool without any extended residence time and be stripped, thus obtaining a high productivity. The RTM mould comprises two or more tool parts which during mould filling and curing define a cavity with the geometry of the part. The mould is always adapted to comprise at least one gate for supply of the liquid polymeric matrix material and at least one deaeration port for evacuating air present in the mould during the mould filling. The positioning and the design of the gate and the deaeration ports are comple¬ tely decisive for achieving a good mould filling. It is common to apply through the deaeration ports a εubatmospheric pressure in the mould in order to increase the pressure difference between the gate and the flow front and hence reduce the time for the mould filling. The use of vacuum, subatmospheric pressure, normally also reduces the content of air inclusions, pores. Typically, the polymeric matrix material is pressed in through the gate at a low pressure, injection pressure, of 0.1 - 1 MPa, which means that the force, the locking force, holding the mould together can be limited. At the same time, it means that the forces acting on the mould surfaces become low whereby good geometrical tolerances can be obtained also with relatively simple mould designs. Taken together, this means that the investment costs for the RTM process are low on a comparison with other processes for forming fibre composites, especially processes for forming high-performance fibre composites. The mould filling is a very critical process step during which it is required that the fibre reinforcement placed in the cavity is filled up completely without air inclusions, pores, remaining in the formed part. In addition, it is required that the mould fill time should be short to attain a high productivity. Complete filling of the cavity and of the so-called free volume in the fibre reinforcement is achieved in the simplest way with simple and symmetrical geometries and with a fibre content in the whole fibre reinforcement which iε evenly distributed in the cavity. For more complex geometries or with a fibre reinforcement with a varying fibre content or a fibre reinforcement unevenly distributed in the cavity, it is often difficult to achieve a good form filling and/or a low pore content. Mould filling, pore content and fill time are thus influenced by a plurality of more or less cooperating parameters, such as injection pressure, subatmospheric pressure in the mould, the viscosity of the polymeric matrix material, its wetting properties relative to the fibre reinforcement, and the sur- faces of the mould, the injection distance, and the permea¬ bility of the fibre reinforcement. Further, the mould filling and the mould fill time can be changed by the choice of mould filling strategy. Two funda¬ mentally different strategies are normally used: A. Point injection, by which is meant that one or more gates is or are placed near the centre of the part and that the flow front propagates from the gate/s in all directions out towards the edges of the part where deaeration takes place through one or more deaeration ports. B. Edge injection, by which is meant that one or more gates is or are placed along one or more of the edges of the cavity via a runner or distribution channel. When the distribution channel iε adapted to supply polymer through gates which are essentially placed along an edge or a side surface, the flow front propagates during the form filling in a direction away from this edge or side surface towards the opposite side surface or surfaces where deaeration takes place. When the distribution channel and the gate are adapted to supply polymer from the edges or side surfaces which constitute the outer limiting surface or periphery of the part, the flow front propagates during the mould filling away from the periphery towards the centre where deaeration takes place. Generally, edge injection provides a shorter fill time than point injection for the same part and with the same proceεε parameters. The permeability of the fibre reinforcement is of decisive importance for the mould fill time. The permeability in the reinforcement iε primarily determined by the volumetric con- tent of fibre in the fibre reinforcement. High fibre con¬ tents, which occur in high-performance polymeric fibre com- poεites, provide considerable resistance for the liquid poly¬ mer compound, the resin, to impregnate the fibre reinforce¬ ment whereby the fill time becomes unacceptably long. When the fill time is long, also the curing is influenced in a negative way since the curing process must be adapted to fulfil the requirement for a low resin viscoεity during eεεentially the entire form fill time. For many resin materials, the requirements for a retained low viscoεity during a long form fill time to enεure good filling and low pore content cannot be combined with the requirements for a rapid curing process in a subsequent curing stage, but usually it is necessary to compromise between theεe require¬ ments. At a constant injection pressure, this usually results in a reduction of the rate of the flow front such that the total form fill time is proportional to the square of the injection distance. In this way, the mould fill time will be unreasonably long for large parts unless a large number of gates can be arranged to shorten the necessary injection distances. However, a large number of gates render the moulds more complicated and expensive. One object of the invented method is to offer a process during which a liquid polymer material with a minimized mould fill time is supplied and fills up a mould defined by a cavity in a forming tool and impregnates a preform of fibre reinforcement arranged in this mould. Further, the invented method provides a complete mould filling with a minimized pore content and a qualitatively repeatable mould filling under controllable conditions. SUMMARY OF THE INVENTION When manufacturing polymeric fibre composite bodies using the RTM technique, a forming tool (a mould) is adapted to define a cavity, whereupon a preformed fibre reinforcement is placed in the cavity before a liquid resin of a low-viscous polymer material is supplied to the cavity. The cavity is filled essentially completely with polymer material whereby the fibre reinforcement is impregnated esεentially completely by the polymer material. When the viscosity of the polymer material has been increased by curing to impart to the formed fibre composite body a stiffneεs which permits the formed fibre composite body to be handled, it is stripped. During the mould filling, the cavity is filled with resin through one or more injection channels centrally arranged in the fibre reinforcement. According to the invented method, the resin is inserted into the mould through a gate arranged on the gate side of the injection channel, and during the mould filling the injection channel is extended in a controlled manner to control the supply of resin such that the flow front of the resin, while filling the cavity and impregnating the fibre reinforcement with resin, propagates esεentially parallel to the axiε of the injection channel through the cavity as the injection channel is extended into the cavity. The invented method is eεpecially εuitable for two typeε of compoεite productε for which the preformed fibre reinforce¬ ment comprise cavitieε which reεult in hollow articleε or parts compriεing both reinforced and non-reinforced portionε, εo-called sandwich designs. Preferably, esεentially cylindri- cal injection channelε are arranged in theεe non-reinforced cavities. The injection channel can be defined as a tube, a liner, with walls permeable to the resin, or be formed by the fibre reinforcement or as a cavity in a core permeable to the resin. The injection channel can be straight or cylindrical but also be provided with contours which follow the geometry of the formed body. This is achieved in one embodiment of the invention by arranging a piston in the injection channel and when reεin during the form filling is supplied through a gate at one end of the injection channel, the piston is withdrawn or pressed out of its inserted position through a hole provided at the other end of the injection channel, whereby the injection channel is extended during the mould filling and the flow front of the resin propagates essentially parallel to the axis of the injection channel. The same effect is achieved when the above-mentioned piεton in another embodiment of the invention iε arranged in the form of a hose which during the mould filling is turned inside out and is withdrawn from the injection channel through a hole arranged at that end of the injection channel which is oppoεite to the gate. According to a further embodiment of the invention, the above-mentioned piston is divided into two parts, with one inner stationary shaft which remains in the cavity during the mould filling to minimize the volume of the injection channel, and one outer εleeve which iε arranged concentric with the above-mentioned εhaft and which during the mould filling iε pressed out or withdrawn, in the manner described above, from a hole arranged at that end of the injection channel which iε oppoεite to the gate. When manufacturing polymeric fibre compoεiteε according to the invented method, the mould fill time is minimized in that during the entire mould fill time the distance from freely floating reεin in the injection channel to the flow front which propagateε through the fibre reinforcement iε εmall and constant, which on a compariεon with conventional RTM technique greatly reduces the mould fill time in connection with: - large and/or long bodies, - bodies with a high fibre content, and - matrix polymers with a high viscosity. In addition, because of the short distance from freely floa¬ ting resin in the injection channel to the flow front through the fibre reinforcement obtained with the invented method, a good control is obtained of the mould filling process, which is thereby controlled by the movements of the piston. With the invented method, a supply of material is obtained which during the mould filling takeε place in a controlled manner from the injection channel or the channels in the interior of the body, which is favourable to avoid air incluεions and other inhomogeneities which may be due to a disturbed material flow through the fibre reinforcement. The invented method is very advantageous for the manufacture of long, hollow products such as high-performance tubes, tubeε with integrated flangeε and for drive εhaftε, aε well aε for integrated manufacture of complex partε εuch as the manufacture of guide surfaceε, including εhaft and attach¬ ment, in one εtep. The invented method iε also very valuable for manufacturing composites with high-viscouε matrix εystemε such as high- temperature resiεtant thermoεetting reεinε, high-εtrength thermosetting reεins and thermoplastic resins. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be explained in greater detail and be exemplified by means of a number of preferred embodiments with reference to the accompanying drawingε wherein Figure 1 shows a method according to the invention where the propagation of the flow front is controlled with the aid of a piston arranged in the injection channel, Figure 2 shows an embodiment where the propagation of the flow front is controlled by turning a hose, arranged in the injection channel, inside out during mould filling and with¬ drawing it from the injection channel, and Figure 3 showε an embodiment through which the volume of the injection channel is minimized during the mould filling. DESCRIPTION OF THE PREFERRED EMBODIMENTS When manufacturing a polymer composite body uεing RTM technique according to one embodiment of the invented method shown in Figure 1, a preformed fibre reinforcement 11 is placed in a forming tool, a mould, 12. In the fibre reinforcement 11 a cylindrical injection channel 13 is arranged. A piston 14 iε inεerted into the injection channel 13 through a hole 15 in the mould 12. In inεerted position the piεton 14 eεεentially fillε up all the volume of the injection channel 13 and can be freely moved in the injection channel 13. A gate 16 is provided in the forming tool 12 and openε out at one end of the injection channel 13, the gate εide 17, whereaε the outlet hole 15 of the piεton is provided at the other end 18 of the injection channel. Deaeration pointε in the form of ports (not εhown) are arranged, aε iε the caεe with conventional manufacture according to the RTM technique, baεed on the geometry of the part. Reεin iε εupplied during the mould filling through the gate 16, provided on the gate side 17 of the injection channel 13, with a conεtant presεure and flow. At the beginn¬ ing of the mould filling operation, the piεton 14 takeε up essentially all of the volume of the injection channel 13. The piston 14 is then moved in a controlled manner such that a flow front 19 is formed which, during the mould filling, propagates with a constant and small advance, X, in relation to the extension of the injection channel 13 controlled by the movement of the piston. The flow resistance in the injec¬ tion channel 13 is small and negligible relative to the flow reεiεtance in the preformed fibre reinforcement 11, which reεults in a small presεure drop in the injection channel 13 during the mould filling. The preεεure difference between the injection channel 13 and the flow front 19 is all the time constant, which is equivalent to a situation where resin is supplied through a gate which eεεentially followε the flow front 19 during its propagation along the injection channel 13 during the mould filling. Thiε great preεεure difference over a εhort diεtance is maintained during essentially the whole mould filling procesε. When eεεentially the whole free volume of the fibre reinforcement 11 has been replaced by polymer material, the mould filling is terminated by retur- ning the piston 14 to its initial poεition, which empties the injection channel 13 with a reversed flow out through the gate 16. Then, the curing stage sets in during which the stiffneεε of the polymer material iε increaεed uεing a tech¬ nique which iε relevant for the polymer material in queεtion. When the formed compoεite body haε reached a sufficient stiffneεε, it iε εtripped using conventional methods for ordinary manufacture by means of RTM technique. As is clear from the embodiment shown in Figure 2, the piston function can be achieved using a hose turned inside out. A preformed fibre reinforcement 21 iε placed in a mould 22. Into an injection channel 23 arranged in the fibre rein¬ forcement 21, a hose 24 is inεerted through a hole 25 in the forming tool 22. At the beginning of the mould filling, the hoεe 24, which at its inserted end 241 is turned inside out and cloεed, essentially fillε up the entire volume of the injection channel 23 by applying an internal overpreεεure to the hoεe 24. A rod or line 242, arranged inεide the hoεe 24, iε attached to the end 241 turned inside out. Since the rod 242 during the mould filling is withdrawn from the hole 25, the hose 24 iε turned inεide and likewiεe withdrawn from the εide the sleeve 34b. The sleeve 34b runs between this sealing layer 34c, which is stationarily secured to the piston 34a, and the piston 34a to presε out the εealing layer so that it seals off the injection channel 33. A gate 36 is arranged in the mould 32 and opens out at one end of the injection channel 33, the gate εide 37, whereaε the outlet hole 35 of the εleeve iε arranged at the other end 38 of the injection channel. During the mould filling resin is supplied, with a controlled preεsure and flow, through the gate 36 arranged on the gate side 37 of the injection channel 33. At the start of the mould filling procesε, the sleeve 34b essentially takes up the whole volume of the injection channel 33. Then the sleeve 34b is moved in a controlled manner such that a flow front 39 is formed, in the manner described above, and propagateε during the mould filling proceεε in a controlled manner essentially parallel to the axis of the injection channel 33 through the cavity. When esεentially the whole free volume of the fibre reinforcement 31 haε been replaced by polymer material, the mould filling iε completed by moving the εleeve 34b back to the initial poεition, which empties the injection channel 33 with a reverεed flow out through the gate 36. Thereafter, the formed compoεite body iε cured and stripped in the manner described above.

Claims: CLAIMS 1. A method of manufacturing bodies of polymeric fibre compo¬ site in a forming tool, a mould, which is adapted to define a cavity, wherein a preformed fibre reinforcement is placed in the cavity whereupon a liquid resin of a low-viscous polymer material is supplied to the cavity such that the cavity is eεsentially filled with polymer material and said fibre reinforcement is impregnated essentially completely by the polymer material during the mould filling whereafter the viscoεity of the polymer material is increased such that the body is given a stiffnesε where the formed fibre compoεite body can be stripped, wherein said resin is supplied to the cavity through one or more injection channels (13, 23, 33, 43, 53) provided in the fibre reinforcement (11, 21, 31, 41, 51) , characterized in that said resin is inserted into the mould through a hole, a gate, (16, 26, 36) provided on the gate side (17, 27, 37) of the injection channel and that the injection channel, during the mould filling, is extended in a controlled manner to control the supply of resin such that the flow front (19, 29, 39, 49, 59) of the resin during the form filling propagates esεentially parallel to the axiε of εaid injection channel through the cavity. 2. A method according to claim 1, characterized in that a piston (14, 24, 34) iε arranged in εaid injection channel,, whereby εaid piston during the mould filling is withdrawn from or presεed out through a hole (15, 25, 35) arranged at that end (18, 28, 38) of the injection channel which iε opposite to the gate side, whereby the extension of the injection channel is controlled and the supply of the resin is controlled such that the flow front (19, 29, 39) of the resin during the mould filling propagates esεentially parallel to the axis of said injection channel through the cavity. 3. A method according to claim 2, characterized in that εaid piston is arranged in the form of a hose which, during the mould filling, is turned inside out and withdrawn from that end of the injection channel which is opposite to the gate side. 4. A method according to claim 2, characterized in that εaid piεton iε arranged in two parts with one inner εtatio- nary shaft (34a) , which remains in the cavity during the mould filling, and one outer sleeve (34b) which iε concentric with εaid shaft and which during the mould filling is presεed or withdrawn through a hole (35) arranged at that end (38) of the injection channel (33) which iε opposite to the gate side (37) .

Inventors: PETTERSSON JOAKIM, SANDLUND ERIK, STROEMBECK ANDERS, PETTERSSON, JOAKIM, SANDLUND, ERIK, STROEMBECK, ANDERS
Applicants: PETTERSSON JOAKIM, SANDLUND ERIK, STROEMBECK ANDERS, SWEDISH INST OF COMPOSITES SIC, PETTERSSON, JOAKIM, SANDLUND, ERIK, STROEMBECK, ANDERS, SWEDISH INSTITUTE OF COMPOSITES, SICOMP
Application year: 1994
Main classification: B29C67
IPC codes: B29C70